ContextLoaderListener的作用就是启动Web容器时，自动装配ApplicationContext的配置信息。
因为它实现了ServletContextListener这个接口，在web.xml配置这个监听器，启动容器时，就会默认执行它实现的方法。
ContextLoaderListener启动的上下文为根上下文，DispatcherServlet所创建的上下文的的父上下文即为此根上下文，可在FrameworkServlet中的initWebApplicationContext中看出。
通常在web.xml中如下配置：
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath*:spring.xml
</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>
层次结构
ContextLoaderListener继承ContextLoader类实现ServletContextListener接口
其中它的主要功能是在ContextLoader中实现，ServletContextListener接口在package javax.servlet中以为servlet的api，
ServletContextListener又继承EventListener，此乃package java.util;中的接口了。
EventListener接口中无任何方法。
ServletContextListener中含有2方法，一个初始化一个销毁。
/**
* Receives notification that the web application initialization
* process is starting.
*
* <p>All ServletContextListeners are notified of context
* initialization before any filters or servlets in the web
* application are initialized.
*
* @param sce the ServletContextEvent containing the ServletContext
* that is being initialized
*/
public void contextInitialized(ServletContextEvent sce);
/**
* Receives notification that the ServletContext is about to be
* shut down.
*
* <p>All servlets and filters will have been destroyed before any
* ServletContextListeners are notified of context
* destruction.
*
* @param sce the ServletContextEvent containing the ServletContext
* that is being destroyed
*/
public void contextDestroyed(ServletContextEvent sce);
ContextLoaderListener初始化容器时序图
在ContextLoaderListener中的contextInitialized方法
**[java]** [view plain](http://blog.csdn.net/zjw10wei321/article/details/40145241#) [copy](http://blog.csdn.net/zjw10wei321/article/details/40145241#)
/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
初始化root跟web上下文，initWebApplicationContext方法在其父类ContextLoader中提供实现。
ContextLoader中initWebApplicationContext方法初始化根上下文
/**
* Initialize Spring's web application context for the given servlet context,
* using the application context provided at construction time, or creating a new one
* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
* @param servletContext current servlet context
* @return the new WebApplicationContext
* @see #ContextLoader(WebApplicationContext)
* @see #CONTEXT_CLASS_PARAM
* @see #CONFIG_LOCATION_PARAM
*/
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//这里判断是否在ServletContext中存在上下文,如果有，说明已载入过或配置文件出错,可以从错误信息中看出
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException(
"Cannot initialize context because there is already a root application context present - " +
"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
// 这里载入根上下文的父上下文
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//这里从web.xml中取得相关的初始化参数,对WebApplicationContext进行初始化
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
}
return this.context;
}
catch (RuntimeException ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
catch (Error err) {
logger.error("Context initialization failed", err);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
throw err;
}
}
根据提供的servlet上下文去初始化Spring的web应用上下文，在构造时使用当前应用上下文或者在web.xml中配置参数contextClass和contextConfigLocation去创建新的上下文。
先判断是否在ServletContext中存在root上下文，如果有，说明已载入过或配置文件出错,可以从错误信息中看出。
通过createWebApplicationContext方法创建web应用上下文，此上下文必定是实现了ConfigurableWebApplicationContext接口，在设置parent for root web application context，在configureAndRefreshWebApplicationContext方法里构造bean工厂和容器里bean的创建，这里就不描述了，下次专门研究这块，最后将跟上下文存入servletContext里，同时根web应用上下文存入到currentContextPerThread，可供后续取出当前上下文，currentContextPerThread = new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);。
ContextLoader中createWebApplicationContext方法创建根上下文
/**
* Instantiate the root WebApplicationContext for this loader, either the
* default context class or a custom context class if specified.
* <p>This implementation expects custom contexts to implement the
* {@link ConfigurableWebApplicationContext} interface.
* Can be overridden in subclasses.
* <p>In addition, {@link #customizeContext} gets called prior to refreshing the
* context, allowing subclasses to perform custom modifications to the context.
* @param sc current servlet context
* @return the root WebApplicationContext
* @see ConfigurableWebApplicationContext
*/
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//这里需要确定我们载入的根WebApplication的类型，
//由在web.xml中配置的contextClass中配置的参数, 如果没有使用默认的。
Class<?> contextClass = determineContextClass(sc);
//contextClass必须实现ConfigurableWebApplicationContext，否则抛异常
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//初始化WebApplication，强转成ConfigurableWebApplicationContext
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
初始化根上下文，
最后返回值需强转成ConfigurableWebApplicationContext。
ContextLoader中determineContextClass方法找到根上下文的Class类型
/**
* Return the WebApplicationContext implementation class to use, either the
* default XmlWebApplicationContext or a custom context class if specified.
* @param servletContext current servlet context
* @return the WebApplicationContext implementation class to use
* @see #CONTEXT_CLASS_PARAM
* @see org.springframework.web.context.support.XmlWebApplicationContext
*/
protected Class<?> determineContextClass(ServletContext servletContext) {
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
else {
contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
try {
return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load default context class [" + contextClassName + "]", ex);
}
}
}
Web.xml中配置了contextClass就取其值，但必须是实现ConfigurableWebApplicationContext，
没有的就取默认值XmlWebApplicationContext。
ContextClass默认值和ContextLoader.properties如下：
/**
* Name of the class path resource (relative to the ContextLoader class)
* that defines ContextLoader's default strategy names.
*/
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
private static final Properties defaultStrategies;
static {
// Load default strategy implementations from properties file.
// This is currently strictly internal and not meant to be customized
// by application developers.
try {
ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
}
}
# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
其中loadParentContext
/**
* Template method with default implementation (which may be overridden by a
* subclass), to load or obtain an ApplicationContext instance which will be
* used as the parent context of the root WebApplicationContext. If the
* return value from the method is null, no parent context is set.
* <p>The main reason to load a parent context here is to allow multiple root
* web application contexts to all be children of a shared EAR context, or
* alternately to also share the same parent context that is visible to
* EJBs. For pure web applications, there is usually no need to worry about
* having a parent context to the root web application context.
* <p>The default implementation uses
* {@link org.springframework.context.access.ContextSingletonBeanFactoryLocator},
* configured via {@link #LOCATOR_FACTORY_SELECTOR_PARAM} and
* {@link #LOCATOR_FACTORY_KEY_PARAM}, to load a parent context
* which will be shared by all other users of ContextsingletonBeanFactoryLocator
* which also use the same configuration parameters.
* @param servletContext current servlet context
* @return the parent application context, or {@code null} if none
* @see org.springframework.context.access.ContextSingletonBeanFactoryLocator
*/
protected ApplicationContext loadParentContext(ServletContext servletContext) {
ApplicationContext parentContext = null;
String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);
if (parentContextKey != null) {
// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
Log logger = LogFactory.getLog(ContextLoader.class);
if (logger.isDebugEnabled()) {
logger.debug("Getting parent context definition: using parent context key of '" +
parentContextKey + "' with BeanFactoryLocator");
}
this.parentContextRef = locator.useBeanFactory(parentContextKey);
parentContext = (ApplicationContext) this.parentContextRef.getFactory();
}
return parentContext;
}
根据在web.xml中配置的locatorFactorySelector和parentContextKey来给根web应用上下设置父上下文，如果没配置的话，父上下文为空。
加载父上下文的主要原因是允许多重root web application contexts作为可共享的ERA context的子节点，或者对EJB可见的去交替共享同样的父上下文。For pure web applications, there is usually no need to worry about having a parent context to the root web application context。这句话明确告诉我们，对于纯粹的Web应用，通常不用担心root web application context的父上下文，也就是没有，为null。
在应用程序如何获取 WebApplicationContext 有多种方式，最简单的就是
1.WebApplicationContext wac = ContextLoader.getCurrentWebApplicationContext();
当前应用的WebApplicationContext就保存在 ContextLoader的currentContextPerThread属性当中
2.基于ServletContext上下文获取的方式
ServletContext sc = request.getSession().getServletContext();
ApplicationContext ac1 = WebApplicationContextUtils.getRequiredWebApplicationContext(sc);
ApplicationContext ac2 = WebApplicationContextUtils.getWebApplicationContext(sc);
WebApplicationContext wac1 = (WebApplicationContext) sc.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
3.还有一些更合适的，基于spring提供的抽象类或者接口，在初始化Bean时注入ApplicationContext
3.1:继承自抽象类ApplicationObjectSupport
说明：抽象类ApplicationObjectSupport提供getApplicationContext()方法，可以方便的获取到ApplicationContext。
Spring初始化时，会通过该抽象类的setApplicationContext(ApplicationContext context)方法将ApplicationContext 对象注入。
3.2:继承自抽象类WebApplicationObjectSupport
说明：类似上面方法，调用getWebApplicationContext()获取WebApplicationContext
3.3:实现接口ApplicationContextAware
说明：实现该接口的setApplicationContext(ApplicationContext context)方法，并保存ApplicationContext 对象。
